Conventional trains typically rely on a pneumatic brake system to provide reliable braking. Although individual brake systems may vary slightly, they typically include a main reservoir, located on a locomotive, that supplies pressurized air through a brake pipe to each rail car connected in series to the locomotive. The brake pipe supplies the pressurized air to a combined auxiliary and emergency reservoir on each rail car. A control valve on each rail car senses pressure in the brake pipe to control actuation of the brake system on each rail car and re-charging of the combined reservoir.
For service braking, an operator slowly vents brake pipe pressure. For example, the operator may vent brake pipe pressure 6-8 pounds per square inch for minimum service reduction and 26 pounds per square inch for full service reduction. The control valve on each rail car senses the reduction in brake pipe pressure and repositions to supply a proportional air pressure from the auxiliary reservoir to a brake cylinder to actuate brake shoes on the rail car. To release the brakes, the operator charges air from the main reservoir to the brake pipe to increase brake pipe pressure. When the control valve on each rail car senses an increase in brake pipe pressure, the control valve repositions to vent air pressure from the brake cylinder to release the brake shoes. The control valve also repositions to allow brake pipe pressure to re-charge the auxiliary reservoir.
For emergency braking, the operator rapidly vents brake pipe pressure. When any control valve senses a sufficiently rapid reduction in brake pipe pressure, the control valve repositions to supply air pressure from the auxiliary and emergency reservoirs to the brake cylinder to actuate the brake shoes. In addition, the control valve repositions to vent brake pipe pressure locally to more rapidly propagate emergency braking to other rail cars in the train.
An important challenge of the brake system is to reliably ensure braking is always available while also reducing or preventing an undesired emergency (UDE) brake application. For example, fluctuations in the brake pipe pressure caused by leaks, temperature changes, flexibility in the brake pipe, length of the brake pipe, and numerous other component and environmental conditions may create a transient pressure fluctuation at an individual control valve that causes an undesired emergency brake application. Compounding this problem, control valves have been historically designed and certified to operate at a nominal brake pipe pressure of 70 pounds per square inch, but many conventional brake systems operate at a nominal brake pipe pressure of 90 pounds per square inch. The control valves may be more sensitive to pressure changes at the higher nominal operating pressure, resulting in more undesired emergency brake applications.
Therefore, the need exists for an improved control valve having enhanced stability to accurately distinguish between pressure changes caused by service braking and other non-emergency conditions from those caused by an emergency braking application. Alternately or in addition, the need exists for an improved method for operating the control valve to reliably distinguish between service braking and emergency braking.